# On Simple Electromagnetism

1. Aug 20, 2011

### Kyoma

1. The problem statement, all variables and given/known data
2 coils are placed side by side without touching. One of them (P) is connected to a galvanometer while the other (S) is connected to a variable resistor and a cell.
There's a switch at S and it is open. The resistance of the rheostat is set at its lowest and then the switch is closed. The resistance of the rheostat is increased gradually to the maximum. Finally, the switch is opened.
The deflection of the galvanometer is the maximum at:

A) At the instant the switch is closed.
B) At the instant the switch is opened.

2. The attempt at a solution

I choose A since current is at its maximum in S and the magnetic field is thus the strongest. However, the answer turns out to be B. Is the answer faulty?

2. Aug 20, 2011

### LiorE

The question is about the *instant* the switch is opened/closed.

At the instant the switch is closed, the current on the S coil is zero (why)? Therefore, the current on the P coil is zero as well. At the instant the switch is opened, there is a current on the P coil, so it's bigger.